Multi-layer capacitors (MLC's) and multi-layer actuators employ high permittivity dielectrics with interspersed high temperature metallizations. In such structures, the formation of low permittivity phases at metallization/ceramic interfaces, low melting temperature phases and interdiffusion of components have created impediments to improved device performance. Those problems have become more significant as industry continues to drive towards reducing internal electrode thicknesses without sacrificing yield or reliability. Combined with the use of flux-sintered dielectrics and Pb-based ferroelectrics, the chemical reactions which may occur between electrodes and the dielectric have thus become more critical.
During co-firing of a dielectric with a metal, the potential for mechanical and chemical interactions and their influence on electrical performance must be carefully considered. Mechanical considerations encompass expansion/contraction due to the oxidation/reduction of the internal electrode, shrinkage mismatch during sintering, and .DELTA.CTE (co-efficient of thermal expansion) during cooling and operation. With regard to chemical interactions, the formation of low permittivity or lossy phases at the interface, low melting eutectic phases, and interdiffusion of components are the major concerns. Such may not only directly impact the presence of undesirable phases, but may also significantly alter microstructural evolution and the defect chemistry of the dielectric. They adversely affect electrical performance and reliability.
BaTiO.sub.3 -based MLC dielectrics with Ag/Pd electrodes, commonly employ fluxes of Bi.sub.2 O.sub.3 and PbO. Not only do those fluxes promote low temperature densification due to transient liquid phase sintering, but they also favorably tailor dielectric properties by entering into solid solution at higher temperatures. Of the two, however, Bi.sub.2 O.sub.3 has been recognized as creating a problem when co-fired with Pd-containing electrodes, i.e., the so-called "bismuth reaction." This reaction has been proposed to occur as: EQU Pd+1/2O.sub.2 .fwdarw.PdO EQU PdO+Bi.sub.2 O.sub.3 .fwdarw.PdBi.sub.2 O.sub.4
or alternately: EQU Pd+Bi.sub.2 O.sub.3 .fwdarw.Pd.sub.x Bi.sub.y +PdO
The problem with PdBi.sub.2 O.sub.4 is that it decomposes at .apprxeq.835.degree. C. to PdO and Bi.sub.2 O.sub.3 ; the PdO immediately reduces to Pd, and the Bi.sub.2 O.sub.3 melts. As Pd.sub.x Bi.sub.y intermetallics melt at very low temperatures and do not wet the dielectric, both reactions may result in electrode discontinuities, microcracking and voids. The addition of Ag or Au or Pt with the Pd has been shown to minimize the bismuth reaction due to the lowered activity of the Pd. It also has been shown that the reaction of PdO and PbO results in PdPbO.sub.2 formation. PdPbO.sub.2 also decomposes at .apprxeq.830.degree. C. to form PdO which immediately reduces, and PbO, which melts at .apprxeq.860.degree. C.
The formation of PbO at a metallization/ceramic interface results in formation of an altered chemistry dielectric that substantially reduces the dielectric properties of the underlying ceramic. Such effects have been observed by Wersing et al. and reported in "PZT-Based Multi-Layer Piezo-Electric Ceramics with AgPd-Internal Electrodes", Ferroelectrics, 1988, volume 87, pages 271-294, at 277.
Accordingly, it is a principal object of this invention to provide an improved MLC structure wherein reactions are minimized between the metallization and the dielectric.
It is a further object of this invention to provide an improved metallization composition for use with Pb-containing dielectrics.
It is a still further object of this invention to provide an improved metallization composition for use with Bi-containing dielectrics.
It is yet another object of this invention to provide an Ag/Pd metallization system that prevents a leaching of Pb from an underlying Pb-containing dielectric.